Flywheels have been used for many years as energy storage devices. They have often been used as power smoothing mechanisms for internal combustion engines and other kinds of power equipment. More recently, flywheels have been recognized as a very attractive energy storage technology for such electrical applications as uninterruptible power supplied utility load leveling systems and electric vehicles.
Modern flywheel energy storage systems convert back and forth between a spinning flywheel's rotational energy and electrical energy. A flywheel energy storage stem includes a flywheel, a motor generator, a bearing system and a vacuum enclosure. The rotating flywheel stores mechanical energy, the motor generator converts electrical or mechanical energy to mechanical or electrical energy, respectively, and the bearing system physically supports the rotating flywheel.
In almost all energy storage applications, whether quick discharge type (power ride-through), where discharge time is measured in seconds, or long-term discharge type (power backup), where discharge time is measured in hours, flywheels directly compete with electrochemical batteries. Two key advantages of flywheels used for electrical energy storage over electrochemical battery systems are its longevity and reliability. Electrochemical batteries, in particular, lead-acid batteries, have short lifetimes, between six months and seven years depending on operating conditions. These batteries require periodic maintenance and can fail unpredictably. In contrast, flywheel energy storage systems are expected to have maintenance-free lifetimes of twenty years.
To achieve a maintenance-free life of many years, the vacuum system that is used to prevent excessive drag and aerodynamic heating of the flywheel must be capable of reliably maintaining an adequate level of vacuum. To date several approaches have been used to create and or maintain vacuum in flywheel systems. The simplest and most widely used approach is to attach an external mechanical vacuum pump to the flywheel chamber. The vacuum pump can then either run continuously or can just turn on when a vacuum gauge indicates that the vacuum has reached an unacceptable pressure. In either case, most vacuum pumps require regular maintenance and have less than twenty-year lives along with the vacuum gauges.
Another method for creating vacuum in a flywheel chamber is to use the flywheel's rotation to drive a molecular drag pump. The flywheel shaft spans two different chambers and a drag pump gear on the shaft pumps gases from the flywheel chamber to the external chamber. The external chamber, which is at a higher pressure, contains physical type getters to trap the gas molecules. Unfortunately, the drag pump is very inefficient at low speeds and could cause the flywheel to take a much longer time to reach full speed if the vacuum level is substantially low. The physical type getters are also found to be not very effective at ambient temperatures either.
Vacuum creation schemes for flywheel systems have also included using pumping action directly from the rotating flywheel itself with the molecules passing through a semi-porous membrane surrounding the flywheel to create the vacuum It is not clear what membrane material could perform this function although, if possible, the structure is likely expensive. Not to mention, this method also suffers from having a lack of vacuum until the flywheel is at high speed. Thus the charging of the flywheel power source would take an extended period of time and much excess power.
To get around the problems of excess low speed drag loss, it is preferable to always have an adequate vacuum regardless of the flywheel speed. Physical type getters such as activated charcoal or zeolites have been proposed for use in sorbing large quantities of gases. Physical type getters trap molecules by having a very porous internal structure in which a small volume of material has an extremely large surface area to which molecules can stick. Unfortunately, as described above, sorption is related to temperature and at room temperature their efficiency is very poor because of the higher energy of the molecules preventing or inhibiting their sticking. They also are not effective for sorbing hydrogen, which may be the largest outgassing component depending on the type of flywheel's construction
To get around the temperature dependence problems of physical getters, evaporable chemical type getters have been proposed. Evaporable getters work by having a base getter material that is continuously heated and vaporized inside the flywheel chamber. The material that is vaporized will condense on the inner walls of the chamber as a clean surface. The clean surface then allows chemical adsorption of gas molecules, which maintains the vacuum. This method can be maintenance-free if enough getter material is included with the flywheel power source. Unfortunately, non-evaporable getters are best suited for very low pressure vacuum because the film can only be deposited at a slow rate. Thus, the gas molecules of a medium pressure vacuum, which is all that is required for a rotating flywheel, would overpower the getter. The getter would also have a limited life.
Compounding the problems of mining the vacuum for an extremely long time inside a flywheel power source, flywheel systems have a couple of uniquely problematic attributes compared with other evacuated devices. Flywheel systems have an unusually large internal surface area for their enclosed volume. The large surface area, which is the result of filling the chamber mostly with the flywheel creates much higher gas evolution For performance reasons, such as reducing the turbulent drag on the flywheel through reduction of the gap between the flywheel and the stationary chamber walls, as well as for shipping and handling purposes, the size of the flywheel chamber is preferably made small. The lower internal volume from the large internal flywheel and small outer chamber means that the internal volume becomes very small. Thus, it does not take much outgassing of molecules before the pressure inside the chamber rises to acceptable levels.